Erasure-proof development of electrostatic images



March 11, 1969 w. A. VAN DEN HEUVEL ETAL 3,432,329

ERASURE-PROOF DEVELOPMENT OF ELECTROSTATIC IMAGES Filed May 18, 1964 Sheet of 4 March 11, 1969 w. A. VAN DEN HEUVEL ETAL 3,432,329

ERASURE-PROOF DEVELOPMENT OF ELECTROSTATIC IMAGES Filed May 18. 1964 Sheet g of 4 Wu), 0%,, hmcuw /iw' ORNEY6 Mar 1969 w. A. VAN DEN HEUVEL ETAL 3,432,329

ERASURE-PROOF DEVELOPMENT OF ELECTROSTATIC IMAGES Filed May 18, 1964 Sheet 3 of 4 March 11, 1969 w. A. VAN DEN HEUVEL ETAL ERASURE-PROOF DEVELOPMENT CF ELECTROSTATIC IMAGES Filed May 18, 1964 Sheet 4 of INVENTOIB 13 0126272 Vamdev/Ywzrel fiawl. 70min qfizgfldiz'rqyeland jVaZ Jfleyald'er;

Q5 an 0+ mm} M ATTORNEY5 United States Patent 292,921/ 63 US. Cl. 117-37 Int. Cl. B44d 1/40 2 Claims ABSTRACT OF THE DISCLOSURE Erasure-proof images are produced on a photoconductive zinc oxide containing recording sheet by developing the electrostatic image on the sheet with an aqueous acidic liquid developer containing leuco compounds from which the corresponding dyes are formed by oxidation and hexacyanoferrate (III) ions. As the developer contacts the electrostatic image in the presence of the zinc ions in the sheet the dyes are oxidized by the hexacyanoferrate (III) ions and the dyes are firmly adsorbed to the formed zinc hexacyanoferrates. Agents may be added to the developing solution to lower the surface tension and to slow the drying rate. The dye and the hexacyanoferrate (III) ions may be applied in two separate steps.

This invention relates to the erasure-proof development of electrostatic images.

It is known that an ink image prepared with an aqueous ink is poorly resistant to a treatment with water if no special precautions are taken. The production of an erasure-proof ink image with such inks thus is a problem that has received considerable attention in the art especially in connection with the detection of falsifieations and improvement in storage stability of important documents.

The development of electrostatic latent images by means of liquid or ink is known for instance from the US. patent specification 2,551,582. It is mentioned therein that an electrostatic latent image can be developed wtih an electrostatically attractable finely divided liquid such as an ink mist. In the British patent specification 902,928 a method for liquid development of electrostatic charge patterns is described, whereby use is made of an electrically insulating liquid, wherein an aqueous phase has been emulsified, and wherein suspended pigment particles according to the principle of electrophores are deposited on the charged areas. According to the method for liquid development of electrostatic charge patterns, described in the Belgian patent specification 610,060, an electrostatic latent image is developed with a developing liquid, the interfacial tension of which in respect of the surface of a solid material carrying an electrostatic charge pattern is influenced by the magnitude and the sense of the electrostatic field strength on this surface in such a way, that this surface is wetted selectively or differently in correspondence with the electrostatic charge pattern.

Further, a method is described in the Belgian patent specification 625,335, whereby an electrostatic charge pattern is formed in or on a surface of a material comprising a photoconductive layer, by traversing this material by an electric field during or after the image-Wise exposure, and whereby this surface is supplied simultaneously in a nondifferen-tial way with a liquid, the surface tension of which with respect to said surface is influenced by the presence and the degree of the electrostatic charge in such a way, that the liquid wets the surface selectively or differentially according to the electrostatic charge pattern.

3,432,329 Patented Mar. 11, 1969 ice For the development of an electrostatic charge pattern color reactions are described in the Belgian patent specification 610,060 between a compound in the developing liquid and a compound in the material to be developed. Parenthetically it may be noted that the formation of a visible image by the reaction of one or more compounds in the surface of the material to be developed with one or more compounds in a liquid phase possesses the advantage in that the formed image is thoroughly anchored in the surface of the material and hence is very resistant to mechanical rubbing 011.. These color reactions can be activated or accelerated by heat and light, and by incorporating catalysts into the material that carries the electrostatic charge pattern and/or into the developing liquid.

It is an object of the present invention to produce visible erasure-proof prints by means of aqueous compositions onto an electrophotographic recording material that can provide zinc ions.

More particularly, it is an object of the present invention to provide liquid developing compositions for use in erasure-proof liquid development of electrostatic charge patterns present in a recording element containing zinc oxide.

It is a further object of the present invention to provide several embodiments according to which erasureproof images of an electrostatic image are produced, which is present on a photoconductive layer containing photoconductive zinc oxide.

These objects are accomplished by image-wise or record-wise applying onto a recording element, preferably a photoconductive zinc oxide containing electrophotographic element, which can provide zinc ions, an aqueous composition (solution or dispersion), which in addition to leuco compounds contains hexacyanoferrate (III) ions. Under the influence of zinc ions the said leuco compounds are quickly oxidized to dyes by hexacyanoferrate (III) ions, while these dyes are firmly adsorbed to the formed zinc hexacyanoferrates.

Leuco compounds which are considered for the production of erasure-proof ink prints according to the present invention belong to the known classes of leuco compounds, from which the corresponding dyes are formed by oxidation. Examples of these leuco compounds are for instance:

A. LEUCO COMPOUNDS OF THE CARBENIUM SALTS (1) Leuco compounds of triarylcarbenium salts also named leuco compounds of triphenylmethane dyes, such /CH Q K CH Q N @M C.

(2) Leuco compounds of the diarylcarbenium salts, also named leuco compounds of diphenylmethane dyes. Diphenylmethane dyes with open structure, such as:

11. H C H C H3 H; C I I 0 Hz 12. H30 H C Hz (3) Leuco compounds of diphenylmethane dyes with ring structure, such as:

B. LEUCO COMPOUNDS OF THE AZENIUM SALTS (1) Leuco compounds of diarylazenium salts without ring closure, such as:

(2) Leuco compounds of diarylazenium salts comprising a ring, which is closed by an oxygen atom, also named Ieuco compounds of oxazine dyes, such as:

HsC\ 0 /CH3 i I H O CH:

CH; H

(3) Leuco compounds of diarylazenium salts comprising a ring closed by a sulphur atom, also named leuco compounds of thiazine dyes, such as:

1 3C\ /CH3 S 11 0 (His =0 1 H C\ /CH: s H O CH (4) Leuco compounds of diarylazenium salts with ring closure by an -NH or -N-(-alkyl)- group such as:

C. LEUCO COMPOUNDS OF CARBONYL DYES Br H V I I g I \W c1 C-OSOaNa a ll The developing liquids, which are used according to the present invention, contain preferably 0.5 to 2% of the above-mentioned leuco compounds and 0.5 to 2% of water-soluble hexacyanoferrate (III) such as potassium hexacyanoferrate (III).

The developing liquids have preferably a pH of 7-2. For acidifying preferably strong acids are used and acids forming soluble zinc salts in order to liberate very fast zinc ions from zinc oxide contained in the recording element.

The developing liquids, which are applied according to the present invention, as liquid ingredient preferably comprise only water, although they may be mixed with other liquids or may comprise thickening agents for adjusting the viscosity.

Further all the developing liquids applied according to this invention may comprise 0.5 to 20% of a substance which influences the surface tension. This Way, the surface tension can be lowered e.g. by the addition of water-miscible organic substances such as methanol, ethanol, acetone, methyl ethyl ketone, acetic acid, hydroquinone, lauryl sulphonates, dodecyl sulphonates, saponine and polyglycol derivatives.

Other usable surface .active substances are given in Textil Hilfsmittel und Waschrohstoife by K. Lindner, Wissensch-aftliche Varlagsgesellschaft m.b.H., 1954, Stuttgart.

Further, up to 10% of substances can be added to the developing liquid which slow down the drying rate of the ink such as glycerol, glycol, and sorbitol, and from 0 till 10% of binding agents which are soluble or dispersable in water such as gum arabic, carboxymethyl cellulose, casein, poly(vinyl pyrrolidone), poly(vinyl alcohol), poly(vinyl acetate), polyacrylate, polystyrene, waxes, silicates and colloidal silicic acid.

In a recording process for obtaining water proof recording images according to the present invention a zinc oxide containing recording material is mordanted by the treatment with hexacyanoferrate (III) ions and by the hexacyanoferrate (11) ions which are formed during the reduction with the leuco compounds. It was found that on the areas of the recording material, which was treated with a solution of hexacyanoferrate (III) ions and a leuco compound, further absorption of cationic dyes is still possible so as to resist erasure, so that the dye image built up with the dyes of the leuco compounds can be a given a different color by an after-treatment with cationic dyes.

Although in fact the method according to the present invention for obtaining erasure-proof ink prints and ink images can be used in the mostly diversified recording and developing techniques, wherein zinc ions providing recording elements are used, this process in its dilferent embodiments is especially suited for the use of liquid development techniques, which can be applied or which are applied to the development of electrostatic charge patterns onto layers containiny zinc oxide, more particularly layers containing photoconductive zinc oxide dispersed in an insulating binding agent.

So, developing compositions used in the present invention can be applied for instance in an electronic recording system described in the U.S. patent specification 2,577,894, wherein recording is carried out by means of a modulated air or gas current, which is charged with ink particles.

According to a preferred embodiment, which can be applied to liquid development techniques for electrostatic charge patterns, an aqueous developing liquid, which contains potassium hexacyanoferrate (III) and one of the leuco compounds mentioned hereinbefore, is deposited image-wise according to one of these techniques onto a photoconductive material containing photoconductive zinc oxide in correspondance with an electrostatic charge pattern. The oxidation reaction of the leuco compound with the potassium hex-acyanoferrate (III) proceeds very rapidly by the zinc ions already present or formed in situ, and the formed dye is firmly adsorbed image-wise to the formed zinc hexacyanoferrates.

When a hydrophobic photoconductive material is developed according to one of the liquid development techniques that are claimed in the Belgian patent specifications 610,060 and 625,335, the liquid image formed by means of a developing liquid as defined above can be covered selectively with an aqueous solution containing a cationic dye. This way, the first dye image formed by oxidation can be covered in an erasure-proof way by a cationic dye, whereby dye patterns are obtained in the most differing color tones and possessing a high color intensity. Self-evidently, also a solution containing hexacyanoferrate (HI) ions can be applied first and thereupon a solution of a leuco compound occasionally together with a cationic dye, or inversely a solution of a leuco compound occasionally together with a cationic dye and thereupon a solution comprising hexacyanoferrate (III) ions.

Of course, multicolor images can be obtained onto one single recording material by successive development with solutions comprising difierent leuco compounds, and occasionally by the subsequent application of aqueous compositions containing cationic dyes possessing a color different from the color of the dyes formed by oxidation of the leuco compounds.

The following examples illustrate the invention.

Example 1 ethanol 500 10% solution of monobutyl phosphate in ethanol 10 10% solution of succinic acid in dimethylformamide l 1% solution of fiuorescein (C.I. 45350) in ethanol The photoconductive dispersion is applied to a paper support coated with aluminum foil in a ratio of 10 sq. :11. per liter by means of a roller coating system and dried.

Cir

The photoconductive layer obtained after drying is charged by a corona to -300 v./cm. by a tension of -7000 v. at the corona wires, and then exposed for 0.7 sec. through a diapositive with a watt lamp at a dis tance of 10 cm. Then the latent image is developed with a developing apparatus as schematically shown in FIG. 1.

The apparatus schematically shown in FIG. 1 comprises an ink roller 10 and a guiding roller 11. The ink roller 10 is moistened with ink from the ink bath 12. A doctor blade 13 regulates the amount of ink supplied. The guiding roller 11 being in contact with the support 15 presses the recording material 14 with its photoconductive layer 16 against the ink roller 10. In the embodiment of the present invention the ink roller 10 is helically grooved. The helical groove has a depth of 0.2 mm. and a top angle of 60. The recording material 14 is led between the guiding roller 11 and the ink roller 10 at a speed of 2 m./min., and hereby the photoconductive layer containing the charge image comes into contact with a liquid meniscus supplied by the helically grooved ink roller 10.

The peripheral speed of the inking roller equals the travelling speed of the photoconductive layer.

The developing ink supplied in this way is prepared by mixing the following solutions A and B.

Solution A:

p,p-Tetramethyldiamine-diphenylmcthane ..g-.. 1 Aqueous hydrochloric acid solution (pH=2) Solution B:

Potassium hexacyanoferrate (III) g 1 Water 100 A photoconductive layer as described in Example 1 is applied to a paper support with as resistivity 10 ohm. cm. After charging the layer as described in Example 1, an untransparent original is episcopically projected for 30 sec. onto the photoconductive layer.

This projection occurs by means of a camera with a Schneider Kreuznach Xenar 4.4 lense having a focal length of 21 cm. and set at the diaphragm aperture 5.6.

The light-source consists of two Sylvania Green 14 watt lamps. The length of the light beam from the light source to an original is 84 cm.

The development occurs as described in Example 1 but with a developing ink obtained by mixing a. 1% aqueous leuco crystal violet solution, acidified with hydrochloric acid up to pH 2, with a 1% aqueous potassium hexacyanoferrate (HI) solution.

A positive erasure-proof blue image is obtained.

Example 3 For the preparation of a photoconductive dispersion, the following products are mixed in a ball mill:

Plexigum P/ 25 (trade name for a 10% solution of a polyacrylate resin in acetone marketed by Rohn &

The dispersion obtained is applied by dip-coating to an aluminum sheet so that the quantity of zinc oxide amounts to 15 g./sq. m.

A latent electrostatic image is obtained on the obtained photoconductive layer in the same way as in Example 1.

Development occurs in an apparatus as schematically shown in FIG. 1. The guiding roller 11, however, is in this case an aluminum cylinder with a diameter of mm. and a length of 25 cm. The ink roller 10 is made of chrome nickel steel 18/8 and has a diameter of 30 mm. and a length of 25 cm. The surface of the ink roller 10 is helically grooved in such a way that the windings touch each other. The groove is V-shaped with a width and depth of 0.5 mm. The ink roller 10 freely rotates in the ink container 12 so that the groove is filled with ink. The ink'isretained in the groove by capillarity.

The photoconductive material 14 is led between the ink roller 10 and the guiding roll 11 at a speed of 3 m./min., the latent image facing the grooved ink roller 10.

In order to increase the contrast between the exposed and the nonexposed areas, a -10 v. tension is applied between the guiding roller and the ink roller.

The developing liquid is obtained by mixing a 1% aqueous solution of 4,4'-bis-dimethylamino-4"-sulphomethyl-triphenylmethane acidified with hydrochloric acid to pH 2, with a 1% aqueous solution of potassium hexacyanoferrate (III). A positive water proof green image is obtained.

Example 4 A photoconductive composition is prepared by mixing the following products:

This composition is applied to an 80 g./sq. m. barytacoated paper. After drying, the thickness of the photoconductive layer amounts to 25,11 Both exposure and development occur as schematically given in FIG. 2. The photoconductive layer 16 of the recording material 18 being in contact with a negative original 19 is exposed for 1 min. with a 75 watt lamp 20 at a distance of 10 cm. and then developed with an apparatus schematically represented by FIG. 2(b).

The guiding roller 21 contacting the support 15 is connected with the positive pole and the smooth ink roller 22 with the negative pole of a 100 volt direct current source 23. The smooth ink roller 22 is made from chromium nickel steel 18/8 and has a diameter of mm. The exposed photoconductive material is led between the smooth ink roller 22 and the guiding roller 21 at a speed of 1 m./min. The developing ink is obtained by mixing a 1% aqueous 4,4'-bis(dimethylamino)-4"-fiuorotriphenylmethane solution, acidified with hydrochloric acid to pH 2, with a 1% aqueous potassium hexacyanoferrate (III) solution.

As shown in FIG. 2(c), a positive blue water proof image 24 is obtained.

Example 5 A photoconductive composition as prepared in Example l is applied to a 70 g./sq. m. glassine paper support in a ratio of 12 sq. m./litre. Both exposure and development as shown in FIG. 3 occur for 6 sec. in contact through a diapositive with a 75 watt lamp at a distance of 10 cm. The ink roller 25 has a diameter of 39 mm, is helically grooved and made from chromium nickel steel 18/ 8. The helical groove 26 has a depth of 0.2 mm. and a top angle of 60.

The guiding roller 27 has a diameter of 30 mm. and is made from aluminum. The ink roller 25 has a speed of 100 t./min. and forms a liquid meniscus 28 against the photoconductive layer 16 to be developed. During development, the ink roller 25 is connected to the positive and the guiding roller 27 contacting the support 15 to the negative pole of a 200 v. direct current source 29.

The developing ink is obtained by mixing a 1% aqueous leuco indigo solution, acidified with hydrochloric acid to pH 3, with a 1% aqueous potassium hexacyanoferrate (III) solution.

A positive erasure-proof light blue image 30 is obtained.

Example 6 2 cc. of a 10% solution of monobutyl phosphate in ethanol and 30 g. of photoconductive zinc oxide powder are added to 100 cc. of a 10% solution of Curnar Resin P-25 (trade name for a coumaron-indene resin marketed by Barrett Chemicals, New York, N.Y., U.S.A.), in methylene chloride. The whole mixture is ground for 4 h. in a ball mill. Just before applying, 4 cc. of a 5% solution of succinic acid in ethanol are added to the photoconductive dispersion. The support to which the dispersion is applied in a ratio of 20 g./sq. m. is a 60 g./ sq. m. glassine paper.

After drying at 40 C., the photoconductive zinc oxide layer is charged, image-wise exposed and developed as described in Example 1, but with a developing substance obtained by mixing 1% acid aqueous solution (pH=2) of Black Indigosol IBL (Cl. 736-71) with a 1% aqueous potassium hexacyanoferrate (III) solution.

Example 7 30 g. of Plexigum P-25 (trade name), g. of photoconductive zinc oxide and 6 cc. of a 10% solution of monobutyl phosphate in ethanol are added to 300 cc. of toluene. This mixture is thoroughly ground in a ball mill and then diluted with 200 cc. of toluene, whereupon 12 cc. of a 5% solution of succinic acid in ethanol are added whilst thoroughly mixing. The photoconductive dispersion is applied to a glassine paper support in a ratio of 20 g./ sq. m. and dried at 40 C.

After drying, the photoconductive layer being in contact with the negative original 19 is exposed for 1 min. with a 75 watt lamp 20 at a distance of 10 cm. as schematically shown in FIG. 4(a) and then developed in an apparatus such as schematically shown in FIG. 4(b). The supply roller 25 is a cylinder made of chromium nickel steel l8/ 8 with a diameter of 25 mm. and a length of 25 cm. The surface of this roller 25 is helically grooved. The grooves 26 are V-shaped and have a depth of 0.5 mm. and a top angle of 60. The developing liquid 31 is obtained by mixing a 1% acid aqueous solution (pH=2) of the leuco 'base having the structure represented by Formula 15:

with a 1% aqueous potassium hexacyanoferrate (III) solution.

The developing liquid 31 is retained in the grooves 26 of the supply roller by capillarity. The guiding roller 27 is an aluminum cylinder with a diameter of 15 mm. and a length of 25 cm.

The exposed photoconductive material 18 having its photoconductive layer 16 in contact with the grooved roller 25, which is moistened image-wise with developing liquid 31 and having its support 15 in contact with the guiding roller 27, is led between these two rollers at a speed of 3 m./min.

A 400 volt direct current is applied between both rollers, by connecting the guiding roller 27 with the positive pole and the supply roller 25 with the negative pole of a direct current source 32.

A positive water-proof ink image is obtained.

We claim:

1. A method of developing an electrostatic image on a recording element containing zinc oxide which comprises the steps of selectively applying to said image a developing liquid which is an aqueous developing composition consisting essentially of hexacyanoferrate (III) ions, a leuco compound of a cationic dye, water and a compound to maintain the pH of the developing composition at 7 or below, the amount of said leuco compound being suflicient to produce a visible coloration when oxidized to dye form and the amount of said hexacyanoferrate (III) ions being sufiicient to oxidize said leuco compound to dye form.

2. The method of claim 1 wherein said developing composition contains about 0.52% of a water-soluble salt of hexacyanoferrate (III) and about 0.52% of said leuco compound.

References Cited UNITED STATES PATENTS 9/1961 Kurz 117-17.5 X 3/1963 Claus 1l717.5 4/1966 Brenneisen et al 96-1 9/1966 Nelson 117--37 X 11/1966 Gesierich et a1. 96-1 3/1967 Klupfel et a1 961 X US. Cl. X.R. 

